The invention relates to an electromagnetic shutter apparatus for a camera, and more particularly, to a shutter control apparatus for an electromagnetically driven shutter which prevents an inadvertent operation during a film winding and a shutter release operation and which permits a desired multiple exposure process.
A shutter mechanism in a conventional camera generally comprises mechanical parts such as levers, cams and the like, and they can be utilized to prevent an inadvertent operation such as a shutter release during a film winding operation, a double exposure, a film winding during a shutter release operation or the inadvertent winding of unexposed film. In a shutter mechanism of the type which is electromagnetically driven, a simple electrical arrangement to turn a switch on and off is all that is required to control the electromagnetic operation. Therefore, it is very difficult to utilize such arrangement to achieve the prevention of above described inadvertent operations.
In an electromagnetic shutter of the type which is contemplated by the present invention, the shutter is directly driven by the electromagnetic action of a permanent magnet and a shutter driving coil. This permits a simple construction, enabling a simplification of the assembling operation and a minimized overall size. On the other hand, movement of the shutter is slow when it is opened, rendering it unsuitable to taking pictures at high speed. While it may be contemplated to increase the source voltage to produce an increased electromagnetic force, there results an increased power dissipation, involving an economical difficulty.
The electromagnetic shutter is controlled by a time constant circuit including a light receiving element and which automatically determines a proper exposure period. Specifically, a shutter drive coil is energized in response to a shutter release operation, and the electromagnetic action between the flux from the coil and the permanent magnet serves to open the shutter. The time constant circuit comprises an integrator circuit including a photometric element and a capacitor and in which values of a diaphragm aperture, film speed, etc., are incorporated. When the output of the circuit, which corresponds in magnitude to the brightness of an object being photographed, reaches a given value subsequent to the release operation, the circuit is effective to deenergize the drive coil, allowing the shutter to close. With this arrangement, a shutter operation may sometimes be impossible when taking a picture in a short exposure period, depending on the setting of the diaphragm aperture and the film speed. Thus, the time during which the drive coil is energized is too short to open the shutter, resulting in a failure to provide an exposure.
A presently used ordinary camera is provided with a self-cocking assembly which is constructed to disable the next shutter release operation before a film winding takes place after a picture has been taken. Thus, the self-cocking assembly represents an interconnection between a shutter release mechanism and a film winding mechanism, and winding of the film thereby charges the shutter. This presents a difficulty when a multiple exposure is desired. In this instance, the connection between the film winding mechanism and a spool on which the film is disposed must be interrupted subsequent to the shutter release operation, and the film winding mechanism once activated for an idle operation under this condition, before a second shutter release operation can be achieved to expose the same frame of the film. However, the operation involved is very cumbersome, and the interruption of the connection between the winding mechanism and the spool may result in a slight movement of the spool, preventing a second exposure to the same exact frame of the film.
A camera incorporating an electromagnetic shutter has no shutter charging mechanism which is interlocked with the film winding operation since the operation of the shutter takes place under the electromagnetic effect. However, there still is a connection between the film winding mechanism and the shutter release mechanism, preventing a next following shutter release unless a film winding takes place beforehand. Consequently, there is a need to provide some means which interrupts the connection between the film winding mechanism and the shutter release mechanism while avoiding an unintended movement of the film in order to permit a multiple exposure.